"专事诽谤的人" can be translated into English as "a dedicated slanderer" or "a professional defamer".
"A dedicated slanderer" emphasizes that the person is fully committed to or specializes in slandering others.
"A professional defamer" highlights that the person engages in defamation as a regular or perhaps even paid activity.
Both expressions convey the idea of someone who habitually or intentionally engages in defamatory behavior.